Scanning Tunneling Microscopy and Scanning Tunneling Spectroscopy Studies of Powdery Palladium/Graphite Model Catalysts

Abstract

Powdery Pd/graphite model catalysts were studied with transmission electron microscopy (TEM), scanning tunneling microscopy (STM), and scanning tunneling spectroscopy (STS), with the later two techniques in air. In contrast to TEM images, STM/STS data are representative of the very surface and are therefore more relevant for an interpretation of the catalytic properties of these kind of catalysts. Besides spherical clusters and layered structures, also platelike and diffuse structures could be distinguished in the STM images. An assignment of the different objects to one of the main elementsgraphite and palladiumwas possible with STS. The I(V) curves on graphite are always much narrower on the V axis than those on palladium, and their slope dI/dV is, thus, larger. Utilizing this distinction, “chemical maps” were determined by taking I(V) data over a grid of surface points and assigning each point the value of the integral under its dI/dV curve. The comparison of such a chemical map with the topographic STM picture of the same area shows that only the spherical cluster structures consist of palladium, in accordance with the TEM pictures. The layered and platelike structures are identified as graphite, and the diffuse structures probably belong to impurities. In any case, the latter consist neither of graphite nor of palladium. A statistical analysis of the cluster morphology reveals that at low Pd loads the clusters are of spherical shape whereas the average width-to-height ratio increases at higher concentrations. The clusters at higher Pd concentrations exhibit a more elliptical shape.

Morphology evolution and local electric properties of Au nanoparticles on ZnO thin films

“…We note, however, that the transition from spherical to an elliptical morphology with the increase of the metal coverage was reported also in Ref. 24 in the case of Pd clusters. Very small clusters ͑Ͻ4 nm͒ were found to mostly exhibit a spherical shape, while for larger particles the lateral size exceeds the cluster height.…”

Thermal Reduction of Pd Molecular Cluster Precursors at Highly Ordered Pyrolytic Graphite Surfaces

“…A mobile particle can act as a new nucleus, and the growth of some particles by coalescence causes them to exhibit elliptical shapes (Figure a,b), as observed for MP(I), versus the spherical shape observed for MP(II) (Figure a,b). Schneider et al report spherical shapes for Pd clusters at low Pd loadings, while, at higher Pd concentrations, more elliptical shapes are exhibited a, suggesting that the surface is in the first and second phases of nanowire formation …”

Platinum and Non‐Platinum Nanomaterials for the Molecular Oxygen Reduction Reaction

“…Cd, Au [125] clusters. Although STS studies in air on nanocrystalline thin films, [115] as well as on carbon, [116] have been carried out.…”